Spot colors for printing are typically specially mixed inks that can be applied by a printing press or printer, and are in contrast with overlayed inks (typically four) which are applied by process printing, e.g. CMYK (cyan, magenta, yellow, black) inks. Spot colors are typically characterized using either CMYK or device independent colors, such as CIELab. Typically only the 100% ink value is characterized, but several values may be used to characterize the spot color. Using more spot values yields more accurate representations of spot colors. When simulating tints, i.e. colors in which the amount of ink is less than 100%, characterization techniques using only information about the 100% ink, the characterizations tend not to produce accurate matches. In order to handle more accurate models of spot colors, models which use some tints besides the 100% ink, specialized raster image processors (RIPs) are used.
In conventional systems it is known to create a certain standard kind of file for use in transmitting color data to be hardcopy-printed or to be proofed. This industry-standardized file contains the image color data, accompanied by a definition of which color space the image data is defined. This color space is often described by a transform called a “profile”.
The image color data can encompass multiple sets of image data, and corresponding multiple color space definitions. This is because multiple images or graphical objects can be in a single print job and even on a single page, and in general the multiple images or graphical objects can be generated from different source devices, each having its respective profile.
The industry standard file is most often a specially formatted Adobe Portable Document File® (PDF). In a PDF document, each graphical object in a page has an associated color-space definition, i.e. the “profile,” which may be said to “describe” how the color looks. The Adobe PDF file format is described in various materials provided by Adobe Systems, Incorporated, including, for example, the PDF Reference: Adobe portable document format version 1.4, Third Edition, 2001 (first printing), ISBN 0-201-75839-3, herein after referred to as the “PDF Reference.” As described in the PDF Reference, the PDF file format includes four special color space families, Pattern, Indexed, Separation and DeviceN.
The color-space definition is most commonly a transform (or transfer function) that can be used to convert the color data in preparation for printing or display on some specific image device, to obtain some specified or desired effect. One common such desired effect is to obtain on one particular image device the closest possible approximation to color as seen or measured on another particular image device. (In this document, “transfer function” is used in its broad sense of any relation between system outputs and inputs.)
The term “color space” is related to the concept of a color language, i.e., a set of coordinates that can be used to describe a color. Some such spaces are simply machine language, i.e. the set of signals used to control a particular image device. These can vary from, e.g., red-green-blue (“RGB”) for control of a typical additive-primary monitor, through CMYK for control of a typical subtractive-primary hardcopy printer. The latter may be a large printing press or a small desk-type incremental printer, and essentially anything in between. These machine languages or so-called “native” color spaces sometimes call for no color management at all; they are simply sent directly to the corresponding image device.
In addition to machine languages, a color space may be of the type sometimes called “perceptual,” or in the printing industry more commonly referred to as “calorimetric”, since what is actually manipulated is almost always visible color as measured rather than as perceived. (For purposes of the present document, the terms “perceptual” and “calorimetric” will be used synonymously.)
In the modern printing industry, conversions from one color space to another, particularly such conversions as are used to take account of the different color-displaying properties of different image devices, are performed by use of one of the various transforms discussed above.